The S63B44 is designed with timed exhaust gas pluses to maintain consistent, temperature, pressure and volume to spool the turbos. The result of this design is reduced turbo lag and better throttle response for a turbo car. I never quite understood how it was designed for such a process.

I came across EE who explained the concept in the new B58 motor (used in the new Z4 I-6 turbo).

As you can see, the timing of the exhaust pulses on a straight six engine would make it so that in a single exhaust manifold running to a single-scroll turbo, one exhaust valve would open up as an earlier exhaust pulse was traveling towards the turbo. That exhaust pulse would then find itself wanting to run back down towards the engine and the newly-opened other valve, rather than just to the turbo alone. Splitting the exhaust into two tracks independently running to a twin-scroll turbo isolates these pulses from each other and gives everything a clean run to the turbo itself.

For the S63, the timing is split between each turbo and it appears based on the manifold design, each bank contributes equally to each turbo.

As explained by head of engineering at M Division

the Twin Scroll turbines separate the exhaust gas flows in the two turbo chargers.

To illustrate this advantage, let’s try the following thought experiment. Let us imagine a very simple setup. Eight cylinders blow through a “hole” on an exhaust gas turbine. The pressure pulse therefore not only hits the turbine, but also spreads into other exhaust gas pipes. I therefore lose energy. This method is called constant-pressure turbocharging: I pump everything together into a large vessel from which the collected exhaust gas passes over the turbine. In our two turbos, however, the Twin Scroll ducts act as a partition in the turbine, so that every exhaust pulse hits the turbine blades directly without getting lost on the way. This is how we can utilise the speed of the gas as well. Not only the volume of the exhaust gas jet, also its dynamics takes effect – and its momentum is converted effectively.

This transfer of exhaust gases offers yet another advantage: when more than one cylinder blows together in a pipe, there is a lot of residual gas there. Consequently, when the exhaust valve is opened, the cylinder cannot fully discharge its exhaust gas. In our motor, the gas flowing out of the cylinder also takes this residual gas with it.